1. Field of the Invention
The present invention relates to a circuit board which is used as, for example, a board for packages for housing a semiconductor element, specifically, to a circuit board which has highly reliable terminal pads for connection with an external electric circuit board via nearly spherical terminals.
2. Description of the Related Art
Hitherto, as a board for packages for housing a semiconductor element such as an LSI (large-scale integrated circuit) has been used a circuit board made of an electrical insulating material such as alumina ceramics. Such a circuit board comprises an insulating base which has a mounting portion for mounting and housing one or more semiconductor elements at nearly the center of a top side thereof; a plurality of metallized wiring layers made of high melting point metal powder of tungsten, molybdenum or the like, which layers are led away from the periphery of the mounting portion toward a bottom side of the insulating base; a plurality of terminal pads which are formed on the bottom side of the insulating base, and to which the metallized wiring layers are electrically connected; and nearly spherical terminals which are made of solder or the like, and attached by brazing to the terminal pads. This circuit board and a lid for sealing the mounting portion form a package for housing a semiconductor element. Then, a semiconductor element is fixedly attached to the bottom of the mounting portion on the circuit board via an adhesive consisting of materials such as glass or resin so as to electrically connect each electrode of the semiconductor element to the metallized wiring layer via a bonding wire, and the lid is joined to the top side of the insulating base of the circuit board via a sealant of glass, resin or the like so as to hermetically seal the semiconductor element inside a package composed of the insulating base and the lid, whereby a semiconductor device is completed as a product.
Such a semiconductor device is implemented on an external electric circuit board by mounting the nearly spherical terminals made of solder or the like which are attached by brazing to the terminal pads on the bottom side of the insulating base, onto a wire conductor of the external electric circuit board which is formed by a resin insulating material and a copper wiring or the like so that the terminal pads are brought into contact with the wire conductor, and thereafter, heating and fusing the nearly spherical terminals at approximately 150-250.degree. C. so as to join the terminals to the wire conductor. Simultaneously, each electrode of the semiconductor element housed inside the package for housing a semiconductor element is connected to an external electric circuit via the metallized wiring layer and the nearly spherical terminal.
The terminal pad on the bottom side of the insulating base of the circuit board forming such a semiconductor device, is shaped into a circle, a polygon with four or more angles, a shape in which a part of a polygon is arc, as disclosed in Japanese Unexamined Patent Publication JP-A 5-183067 (1993), in order to attach the terminal made of solder or the like as a nearly spherical protrusion to the terminal pad, whereby the terminal is formed as a nearly spherical protrusion owing to the surface tension of the solder to be attached.
However, in the case where such a conventional circuit board is connected to an external electric circuit board via nearly spherical terminals, a coefficient of thermal expansion of the insulating base made of alumina ceramics or the like is about 4.times.10.sup.-6 to 7.times.10.sup.-6 /.degree. C., whereas the external electric circuit which is generally made of a resin insulation material such as a glass epoxy, has a large coefficient of thermal expansion of 1.times.10.sup.-5 to 4.times.10.sup.-5 /.degree. C. There is a large difference in thermal expansion coefficient between them. Therefore, in the case where both of the insulating base of the circuit board and the external electric circuit board are subjected to repeated temperature cycles, which occur due to the semiconductor element which heats during operation and cools off to an ambient temperature when being halted, a large thermal stress is generated between the both due to a difference in thermal expansion coefficient. This thermal stress acts between the terminal pad and the nearly spherical terminal, brings a fatigue to a joint of them, and finally causes an separation and failure at the joint. As a result, the conventional circuit board has a problem that each electrode of a semiconductor element mounted on the circuit board cannot be electrically connected to a designated external electric circuit board for a long time period.